This invention relates to a brake drum particularly adapted for motor vehicles and being reinforced with an embedded reinforcement member to provide a composite structure.
Brake drums used for motor vehicles such as heavy duty trucks are typically formed by casting grey iron and machining the casting in areas where precision dimensions and surfaces are required. Although iron brake drums perform satisfactorily, designers of braking systems are constantly striving for enhanced performance, lower cost, increased fatigue life and lighter weight. One particular shortcoming of conventional all-iron drums is their susceptibility to heat checking and crack formation which can ultimately lead to failure of the drum.
As a means for providing an improved brake drum, composite structures are known in which steel is incorporated into an iron brake drum for reinforcement. The brake drum according to U.S. Pat. No. 4,858,731, which is commonly assigned to the assignee of the present invention, employs a cage-like reinforcement assembly made from steel wire which is cast in place to be substantially embedded within a grey iron brake drum. Locating wires are provided to position the reinforcement structure with respect to the mold cavity during casting. Since the steel material of the reinforcing assembly has a considerably higher modulus of elasticity than grey iron, the reinforcement increases the strength of the composite drum structure, thus decreasing mechanical deflection in response to loading. Although this brake drum makes improvements over previous drums, the locator wires of the reinforcement structure locate on both of the two mold halves when positioning the reinforcement structure. In this regard, the locator wires cross the parting line of the mold and present problems in terms of maintaining tolerances in the positioning of the reinforcement structure relative to the machined, loading or friction surface of the drum.
Additionally, the interior ends of the locator wires were required to have precise diameters in that this end was required to contact both mold halves. Furthermore, Applicant's prior design tended to cause sand from the casting molds to be scraped free as the two mold halves are put together, leading to imperfections in the final product.
Furthermore, the outer ends of the locator wires were rigidly positioned radially in the casting mold. During casting, as the reinforcement rings enlarge due to thermal expansion, the reinforcement rings would tend to bulge outward radially between the locator wires resulting in an uneven spacing of the reinforcement wires from the machined friction surface of the brake drum.
In accordance with this invention, an improved composite brake drum is provided which exhibits a number of significant benefits over previous cast brake drums. The brake drum according to this invention employs a cage-like reinforcement assembly, preferably made from steel wire, which is cast in place to be substantially embedded within a grey iron brake drum. Specifically, the present invention offers an improvement in the locating and positioning of the reinforcement assembly relative to the machined, loading or braking surface of the drum. The locating means of the present invention accurately positions the reinforcement assembly with respect to one mold half during casting and therefore, neither crosses the parting line of the mold nor requires spanning the separation of the mold halves at the locating end. Thus, properly positioned, the reinforcement assembly eliminates the tolerance problems which lead to reduced drum life.
The locating means also allows for easy reforming and fine tuning of the reinforcement assembly prior to mounting within the mold. This further assures accurate positioning of the reinforcement assembly.
The present invention is additionally beneficial in that it allows green casting sand, which has been scraped or dislodged from the mold during positioning of the reinforcement assembly, to be cleared from the mold cavity before the mold is closed for actual casting. With the present reinforcement assembly contacting only one mold half, upon closing of the mold, additional casting sand will not be scraped from the second mold and the mold cavity will remain free from contaminants, upon closing of the mold. With the free or loose sand removed, the porosity of the casting is reduced and the strength of the drum proportionally increased.
The reinforcement assembly also reduces the generation of surface checks and cracks which can propagate and ultimately cause mechanical failure of the brake drum. The increased strength of the composite further enables a reduction in the quantity of iron that is required to produce a brake drum of given strength, thus resulting in a lighter weight brake drum structure. The reinforcement assembly further results in the reinforcing sections being positioned close to the machined friction surfaces of the drum within tolerances required for the most advantageous structural efficiency. The axial aspect of the reinforcing assembly serves to reinforce the brake drum across the entire depth of the friction surface. Significantly, the composite brake drum according to this invention can be fabricated using conventional sand casting processes with minimal variations, thus saving the cost of retooling. Due to the fact that the metal reinforcing sections of this invention are distributed, relatively small diameter wires can be used which in turn enable the wires to be rapidly heated to temperatures near those of the molten iron being poured into the casting mold. Thus, good fusion between the iron and embedded steel reinforcement is promoted and casting cycle time is reduced.
To overcome the problem of uneven spacing of the reinforcement rings caused by thermal expansion, the one end of the locator wires responsible for positioning of the reinforcement structure relative to the radially outer surface of the mold cavity is formed with a spring section. This allows the locator wires to move radially outwardly and the reinforcement rings to expand radially outwardly at the location of the locator wires as well as circumferentially therebetween. The result is a circumferentially even movement of the rings radially in response to thermal expansion. Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.